The range of applications of image-forming apparatus such as electrophotographic copiers is increasing in recent years, and the market is coming to demand a higher level of image quality. In particular, in the production of business documents or the like, the image-inputting technique and the technique of forming a latent image have been developed and a richer variety of character types and a higher degree of character fineness have come to be used or attained in output. In addition, the spread and development of presentation software have led to a desire for the reproducibility of latent images of extremely high quality which give printed images having few defects and little blurring. Especially in the case where an electrostatic latent image on the latent-image carrier as a component of an image-forming apparatus is an image made up of lines of 100 μm or thinner (about 300 dpi or higher), use of conventional toners having a large particle diameter as a developer generally results in poor thin-line reproducibility. Such conventional toners are still insufficient in the clearness of line images.
In particular, in image-forming apparatus employing digital image signals, such as electrophotographic printers, a latent image is constituted of an arrangement of given dot units, and a solid-image area, half-tone area, and light area are expressed by changing dot density. However, when a toner is not disposed faithfully on the dot units and the position of the dot units does not coincide with the position of the actually disposed toner, the result is a problem that the toner image does not have the gradation corresponding to a dot density ratio between black and white areas of the digital latent image. Furthermore, in the case where resolution is to be improved by dot size reduction in order to improve image quality, it becomes more difficult to faithfully develop a latent image constituted of microdots. There surely is a tendency in this case that an image which has high resolution and poor gradation and lacks sharpness is obtained.
Moreover, because of the advent of a blue laser, dot sizes in electrostatic latent images are expected to further decrease in future. There is a desire for an image formation technique applicable to such trend.
Under these circumstances, developers intended to improve image quality have been proposed which have a regulated particle size distribution so as to attain improved reproducibility of microdots. Patent document 1 proposes a toner having an average particle diameter of 6-8 μm. It was attempted therein to develop a latent microdot image with satisfactory reproducibility by reducing particle diameter. Patent document 2 discloses a toner having a weight-average particle diameter of 4-8 μm and comprising toner base particles which include 17-60% by number toner base particles having a particle diameter of 5 μm or smaller. Patent document 3 discloses a magnetic toner including 17-60% by number magnetic toner base particles having a particle diameter of 5 μm or smaller. Patent document 4 discloses toner base particles having a toner particle size distribution in which the content of toner base particles having a particle diameter of 2.0-4.0 μm is 15-40% by number. Patent document 5 describes a toner in which particles of 5 μm or smaller account for about 15-65% by number. Patent document 6 and patent document 7 disclose toners of the same kind. Patent document 8 describes a toner which includes 17-60% by number toner base particles having a particle diameter of 5 μm or smaller, 1-30% by number toner base particles having a particle diameter of 8-12.7 μm, and up to 2.0% by volume toner base particles having a particle diameter of 16 μm or larger, and which has a volume-average particle diameter of 4-10 μm and has a specific particle size distribution with respect to the toner particles of 5 μm or smaller. Furthermore, patent document 9 describes toner particles which have a 50%-volume particle diameter of 2-8 μm and in which toner particles having a particle diameter of 0.7×(50%−number particle diameter) or smaller account for 10% by number or less.
However, those toners each contain particles of 3.56 μm or smaller in a large amount in terms of % by number exceeding the upper limit which is the right side of the expression (4) according to the invention. This means that with respect to relationship between particle diameter and fine powder, the proposed toners each are a toner in which a fine powder remains in a relatively large amount in toner particles having a given particle diameter. Because of the proportion of a fine powder which is still high, such toners have had the following unsolved problems. When such a toner is used in development techniques which require a toner having the ability to be quickly electrified, such as the ability to be instantaneously charged by friction, as in, in particular, nonmagnetic one-component development, then some particles remain insufficiently charged. Because of this, troubles arise such as toner particle falling or toner particle scattering from the developing roller, the residual-image phenomenon (ghost) in which a printing history in the first cycle is reflected in the developing roller in the second and succeeding cycles to selectively increase/reduce image density, and the fouling of printed images due to a drum cleaning failure or improper toner layer formation on the developing roller.
In recent years, there is a desire for life prolongation and high-speed printing besides the market demand for image quality. However, the conventional toners do not fully satisfy these requirements. Toners having a high fine-powder content like the conventional toners further have had the following problem. With the progress of continuous printing, the fine powder fouls members to reduce, e.g., toner-charging ability, resulting in poor image reproduction. When such a toner is used in a high-speed printer, there also has been a problem that toner dusting occurs considerably.
For providing high-image-quality printing, it is necessary that a toner should have a narrow particle diameter distribution. This is because when a toner contains coarse particles, this toner has a broad charge amount distribution and this results in the phenomenon called “selective development”. The “selective development” is a phenomenon in which when a toner having a broad charge amount distribution is used, only the toner particles having a charge amount necessary for development are used and consumed for development in copying. Consequently, satisfactory images are obtained in the initial stage of copying. However, with the progress of continuous copying, the density gradually decreases or toner particles having a larger diameter come to be used to give grained images. A toner which undergoes such a phenomenon is regarded as a toner having poor unsusceptibility to selective development. Furthermore, coarse particles having a small charge amount tend to considerably reduce a guaranteed life in terms of number of prints. Patent document 10 discloses a toner containing a large amount of coarse particles, i.e., having a coefficient of variation in number of 24.2%. Such a toner is unsuitable for stably providing high-resolution images. Patent document 11 does not indicate a narrow particle size distribution.
For providing high-image-quality printing, it is necessary to give attention to the transferability of toners. A toner having high transferability is such a toner that toner particles disposed on a latent image on a photoreceptor are transferred highly efficiently to an intermediate transfer drum or paper or that toner particles are transferred highly efficiently from an intermediate transfer drum to paper. Patent documents 12 to 14 disclose pulverization toners, which are thought not to have a high degree of circularity because of the production steps. These pulverization toners are unsatisfactory from the standpoint of providing high-image-quality printing.
In an electrophotographic apparatus, a toner which has developed an electrostatic latent image formed on the electrostatic-image holding member is transferred to a receiving material, e.g., paper. There are cases where the toner is transferred from the electrostatic-image holding member to a sheet of paper not directly but indirectly through an intermediate transfer material. In this transfer part, the toner is not wholly transferred from the electrostatic-image holding member and a small proportion thereof remains as an untransferred toner on the electrostatic-image holding member. Consequently, a cleaning part is necessary in which the untransferred toner is removed from the electrostatic-image holding member after transfer.
In this cleaning part, the cleaning blade method has been frequently employed hitherto. Namely, in this method, a cleaning blade made of a material having a relatively low modulus, such as, e.g., a urethane rubber, is brought into contact with the electrostatic-image holding member to wipe off the untransferred toner based on the movement of the cleaning blade relative to the electrostatic-image holding member. Although a tip ridgeline of the cleaning blade is in contact with the electrostatic-image holding member to dam up the untransferred toner, the ridgeline is finely vibrating when viewed microscopically. The tip ridgeline elastically deforms, in the state of adhering to the electrostatic-image holding member, with the movement of the electrostatic-image holding member due to the force of resistance of static friction with the electrostatic-image holding member, and is released to recover the original shape when elastic repulsion exceeds the force of resistance of static friction. This tip ridgeline which has recovered the original shape adheres to the electrostatic-image holding member and elastically deforms again. The tip ridgeline repeatedly undergoes the microscopic vibration, which includes those steps. This phenomenon is called “stick-and-slip”.
Even when stick-and-slip occurs in conducting cleaning for toner removal, the untransferred toner dammed up and collected is usually prevented from leaking out through the gap between the cleaning blade and the electrostatic-image holding member. However, it is difficult in some cases to completely dam up slippy particles such as small particles or particles having a high average degree of circularity.
Completely removing small particles necessitates strict control regarding component position accuracy, etc. When particles which are small as compared with the average particle diameter are contained in a large amount, there is a higher possibility that an untransferred toner might pass through the cleaning blade. Although toners are shifting from pulverization toners to wet-process toners in recent years, wet-process toners have a smoother surface and a higher average degree of circularity than pulverization toners and are hence more apt to pass through. Even among pulverization toners, there recently are many toners to which a high average degree of circularity has been imparted by smoothing the surface with heat or through mechanical processing. Such pulverization toners also are apt to pass through. Consequently, there currently is an increasing desire for an image-forming apparatus in which toner particles are less apt to pass through.
In the stick-and-slip phenomenon, the width and period of the vibration depend on the force of resistance of static friction between the cleaning blade and the electrostatic-image holding member and on the force of resistance of dynamic friction therebetween (which relates to the rate at which the cleaning blade recovers the original shape thereof). There are even cases where at a given vibration width and a given vibration period, toner particles having a specific particle diameter, specific shape, or specific degree of slippiness are especially apt to pass through. Such phenomenon in which specific particles are especially apt to pass through is exceedingly difficult to deal with theoretically, and a sufficient knowledge has not yet been obtained on what combination of a toner, an electrostatic-image holding member, and a cleaning blade attains the state in which toner particles are less apt to pass through.
Meanwhile, in view of the market demand for toner particle diameter reduction for higher resolution, it is necessary to provide a technique which attains stable cleaning performance. Although the necessity of this technique is becoming higher because of the advent of wet-process toners and pulverization toners having a smooth surface as stated above, there has been no satisfactory technique.
Among evaluation items for printed images is gloss. Gloss reflects the degree of glossiness of an image. In some cases, higher values of gloss such as those required of photograph image quality are preferred. However, it is desirable to avoid excessively high gloss because too high gloss values result in image glittering.
For stably providing high-resolution images, it is necessary to use a toner having excellent electrification characteristics. Although a technique for incorporating a charge control agent into a toner is known, it has been difficult to incorporate a charge control agent into a toner having a small particle diameter.
Patent Document 1: JP-A-2-284158
Patent Document 2: JP-A-5-119530
Patent Document 3: JP-A-1-221755
Patent Document 4: JP-A-6-289648
Patent Document 5: JP-A-2001-134005
Patent Document 6: JP-A-11-174731
Patent Document 7: JP-A-2001-175024
Patent Document 8: JP-A-2-000877
Patent Document 9: JP-A-2004-045948
Patent Document 10: JP-A-2003-255567
Patent Document 11: WO 2004-088431
Patent Document 12: JP-A-7-98521
Patent Document 13: JP-A-2006-91175
Patent Document 14: JP-A-2006-119616